Device and method for moulding a grooved structure into a tubular workpiece

ABSTRACT

According to the invention, a press roller device (1) for molding a grooved structure inside a tubular workpiece (7) has several press rollers (4,5,6), said press rollers rotating around a form tool (2) and being staggered axially in relation to one another. At least two of the press rollers (4,5) are driven in opposite directions to each other. This arrangement prevents the workpiece from twisting to a large extent.

The invention relates to a pressing roller device and a method formoulding a grooved structure, for example a thread or a toothing, intothe inner surface of a tubular workpiece.

Devices are known for the manufacture of pipes with internal toothing orinternal threads, which have a round shaping tool with an externaltoothing, on to which a tubular workpiece is pushed. Pressure is exertedon to the attached pipe from the outside with a pressing tool, with theresult that an internal thread corresponding to the shaping tool isformed in its interior. The pressing tool is moved in the longitudinaldirection of the pipe with an advancing movement, with the result thatthe same pressure is exerted on to each point of the outer surface.

In the so-called cylindrical pressing methods, smooth cylindrical pipesof relatively small wall thickness are made from a relatively shortworkpiece with a thick wall thickness. In this procedure the workpieceis subjected to a rotating extrusion moulding procedure. The workpieceis attached to a cylindrical shaping tool provided with a thread or atoothing, while, on the outside, several press rollers rotate about theshaping tool and the workpiece and in this way press the workpiece intothe contouring of the shaping tool and in the process extend it with anadvancing movement. An extrusion-deformation of the metal takes placebetween the shaping tool and the press rollers, with the original wallthickness of the workpiece decreasing and the length increasing as aresult.

In DE-OS 24 20 014 a so-called flow turning process is described. Aworkpiece located on a tool mandrel is pressed by at least threepressing rollers rotating about the workpiece and subject to an axialadvancing movement, against the shaping tool which has an externalthread or a toothing, with the result that an internal thread or atoothing is formed in the workpiece. The driven pressing rollers arelocated in a radial plane of the shaping tool, i.e. the three identicalpressing rollers engage the same circumferential region of theworkpiece. Because the shaping tool is rotated relative to the pressingrollers, a relatively large torsional force acts on the workpiece, thisforce leading to a twisting of the workpiece during the plasticdeformation by way of the pressing rollers. The torsion of the workpieceleads to a strain on the shaping structure of the tool, with the resultthat the service life of the shaping tool is considerably reduced.Shearing of the shaped structure can even possibly result.

The object of the invention is to improve the manufacture of tubularworkpieces with an inner grooved structure to the extent that thetorsion of the workpiece and therefore the strain on the shaping tool isconsiderably reduced.

In tests it has been found that the torsion of the workpiece can bealmost completely prevented if the entry angle of the pressing rollerwhich has engaged last is greater than the entry angle of the forwardpressing rollers.

The entry angle of the last pressing roller is preferably at least 30°,the entry angle being, in particular, approximately 50% larger than thatof the first pressing roller.

Further advantages developments of the invention will be explained inmore detail in connection with the drawings.

FIG. 1 shows a perspective view of a pressing roller device.

FIG. 2 shows a rear view according to the arrow II in FIG. 1.

FIG. 3 shows a longitudinal section through the pressing roller device.

The pressing roller device 1 shown in FIG. 1 has a cylindrical shapingtool 2, to the outer surface of which a helical grooved formation 3 isapplied. The shaping tool 2 is formed of hardened steel or hard metal.Three pressing rollers 4, 5, 6 rotate about the shaping tool 2. Thepressing rollers 4, 5, 6 are spaced slightly axially apart from eachother, with the result that the first pressing roller 4 in the advancingdirection is the first one to meet a workpiece 7 located on the shapingtool 2. The pressing rollers 4, 5, 6 each have a spacing of about 0.1 to5 mm, this being small in contrast to a roller width of, for example, 70to 80 mm. In this respect, offset does not mean that the rollers have aspacing between them, but that, for example, the respective points ofcontact of the pressing rollers 4, 5, 6 are staggered by 0.1 to 5 mm,i.e. that the circular paths of the pressing rollers 4, 5, 6 overlap.

The workpiece 7 is hollow-cylindrical in the non-machined starting stateand is attached to the end of the shaping tool 2, with its innerdiameter being dimensioned in such a way that it rests on the raisedgrooved structures 3. For the shaping of the workpiece 7, the shapingtool 2 rotates together with the workpiece 7 which is secured to it, andthe pressing rollers 4, 5, 6, which are secured to a feeding carriage,not shown here, are subjected to a feeding displacement in the axialdirection. The pressing rollers 4, 5, 6 are pressed radially against theworkpiece 7, with the result that the latter is plastically deformed.

In FIG. 1 the pressing rollers 4, 5, 6 are rotated in the drawing planefor a better representation. The actual arrangement is shown in FIG. 2.The three pressing rollers 4, 5, 6 are arranged equidistantly in termsof the circumference, i.e. between them there is a circumferential angleof 120° in each case.

The pressing rollers 4, 5, 6 are rotatably mounted, with the threepressing rollers 4, 5, 6 being driven in the same direction as theshaping tool 2 (in FIG. 2 in anticlockwise direction). This leads to thematerial of the workpiece 7, which has become plastic under the contactpressure, for example of the pressing roller 4, being slowed down to acertain extent between the pressing roller 4 and the shaping tool 2 andin this way being pressed better into the roller structure 3.

FIG. 3 shows the three pressing rollers 4, 5, 6 in the plane of thedrawing in a better representation. In fact, the three pressing rollers4, 5, 6 are actually staggered by 120° to each other in terms of thecircumference. For a better representation, the offset w4, w5, w6between the pressing rollers 4, 5, 6 is shown to be greater than it isin reality.

The first pressing roller 4 is the first one to come into contact withthe workpiece 7. It abuts the workpiece 7 with a conical pressingsurface 8. The wall thickness of the workpiece 7, proceeding by way ofaxial advancing movement from an original wall thickness S₀, is reducedby the first pressing roller 4 by the thickness d₄. In this respect, thefirst pressing roller 4 runs on helical paths over the surface of theworkpiece 7. The advancing movement and the rotating speed with whichthe pressing roller 4 rotates the workpiece 7 are coordinated with eachother in such a way that the pressing roller 4 covers the entire surfaceof the workpiece 7. The entry angle α₄ of the first pressing roller 4lies in an angle range of 5 to 30° and preferably amounts to 20°. Theentry angle is the angle between the pressing surface 8 and the outersurface of the workpiece 7. The pressing surface 9 of the secondpressing roller 5 has the same geometry. The pressing surface 10 of thethird pressing roller 6 extends at an entry angle α₆ which is largerthan the entry angles α₄, α₅ of the first pressing rollers 4, 5. Theentry angle α₆ lies in the range of 10° to 40° and preferably amounts to30°.

A transition region 11 continues from the pressing surface 10, thetransition region passing over into an exit surface 12 of the pressingroller 6. The transition region 11 has a radius R₆ which correspondsapproximately to the starting wall thickness S₀ plus half the end wallthickness S₁ of the workpiece 7 (R₆=S₀+0.5 S₁). The first and secondpressing rollers 4, 5 also have a respective transition region with theradii R₄ and R₅, with these radii corresponding to the radius R₆. Theexit surfaces extend at an exit angle β₄, β₅,β₆ with respect to theouter wall of the workpiece 7. The exit angles β of the pressing rollers4, 5, 6 are of the same size and lie in angle range of 0° to 15°,preferably 3° to 5°.

The offset w4, w5, w6, that is to say the respective axial spacingbetween the pressing rollers 4, 5, 6, amounts to 0.1 to 5 mm in eachcase.

The pressing rollers 4, 5, 6 have a variable radial spacing from theshaping tool 2, or from the workpiece 7. The first pressing roller 4 hasthe largest spacing because it works on the workpiece 7 first. At theexit-side end of the pressing surface 8 of the first pressing roller 4,the original wall thickness S₀ of the workpiece 7 is reduced by theamount d₄. The input-side end of the pressing surface 9 of the secondpressing roller 5 now engages in this radial spacing S₀-d₄. The wallthickness is reduced by the amount d₅ by means of the pressing surface 9of the second pressing roller 5. The last pressing roller 6 reduces thewall thickness by the amount d₆ until the desired target wall thicknessS₁ of the workpiece 7 is reached. The wall thickness of the workpiece 7is therefore reduced from the original wall thickness S₀ to the targetwall thickness S₁. The wall thickness reduction d is composed of theindividual reductions d₄, d₅, d₆, with each individual reductionamounting to 0.2 to 0.4 times the total reduction d.

The shaping of the inner grooved structure also takes place in sections.At the beginning of the pressing roller operation the workpiece 7 restson the raised grooved structure 3 of the shaping tool 2. The wallthickness S₀ of the workpiece 7 corresponds to the following formula:

S₀=2×S₁+2.2×m

with S₁ being the final wall thickness and m being the toothed module ofthe shaping tool 2 which corresponds to the flank clearance of twoadjacent grooves divided by. The first pressing roller 4 presses thematerial of the workpiece 7 somewhat into the grooved structure 3 of theshaping tool 2. The pressing roller 5 presses the material further intothe grooved structure 3, while the last pressing roller 6 fills thegrooved structure 3 completely with the material of the workpiece 7 andsets the desired target wall thickness S₁.

What is claimed is:
 1. A pressing roller device for molding a groovedstructure into a tubular workpiece, the device comprising a tubularshaping tool having an externally grooved structure, at least threepressing rollers which rotate relative to the shaping tool and AREspaced around a circumference of the tubular shaping tool, the at leastthree pressing rollers being axially displaceable in an advancingdirection and driven in a rotating manner, the pressing rollers havingconical pressing surfaces extending at a respective entry angle with thetubular workpiece, wherein the pressing surface of the last pressingroller have a larger entry angle than the pressing rollers forward oflast pressing roller.
 2. The pressing roller device according to clam 1,characterized in that the pressing surface of the last pressing rollerhas an entry angle of about 30° and the other pressing rollers having anentry angle of about 20° in each case.
 3. The pressing roller deviceaccording to clam 1, characterized in that the pressing rollers arestaggered slightly axially with respect to each other, with the distancebetween the pressing surfaces of the pressing rollers and the shapingtool increasing in the advancing direction.
 4. The pressing rollerdevice according to claim 3, characterized in that an axial spacing froma forward edge of one of the pressing rollers to a forward edge of anadjacent pressing roller amounts to 0.1 to 5 mm.
 5. The pressing rollerdevice according to claim 1, characterized in that each pressing rollerhas transition region between the pressing surface and an exit surface,a radius of the transition region corresponding to a starting wallthickness (S₀) plus half an end wall thickness (S₁) of the workpiece. 6.The pressing roller device according to claim 1, characterized in that astarting wall thickness S₀ of the workpiece corresponds to the followingformula: S₀=2×S₁+2.2×m with S₁ being the material thickness over thegrooved structure o the finished workpiece and m being the toothedmodule of the shaping tool.
 7. The pressing roller device according toclaim 1, characterized in that the pressing surface of the last pressingroller has an entry angle of 10-40° and the other pressing rollers havean entry angle of 5-30°.
 8. A method for molding a grooved structureinto a tubular workpiece with the use of a device which has at leastthree axially staggered pressing rollers rotating about a shaping tooland spaced around a circumference of the shaping tool, the methodcomprising: attaching the workpiece, which has a starting wall thicknessS₀ greater than a target wall thickness S₁, on to the shaping tool;rotating the shaping tool; advancing of the pressing rollers along theworkpiece in an advancing direction, with the last pressing roller inthe advancing direction having a pressing surface, the entry angle withthe tubular workpiece of which is larger than the entry angles of thepressing rollers forward of the last processing roller.